Summary of Significant Accounting Policies (Restricted Cash) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Restricted cash	$ 8	$ 41
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	71	1,091
Short-term restricted cash	1	40
Long-term restricted cash	7	1
Total cash, cash equivalents, and restricted cash	79	1,132	$ 133	$ 114
Southern California Edison
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	39	515
Total cash, cash equivalents, and restricted cash	$ 39	$ 515	$ 61	$ 40